1. Field of the Invention
The present invention relates to a radio base station, a relay station, using radio communication, and a radio communication method. The present invention is especially advantageous when it is applied to a case where a radio communication system prescribed by IEEE802.16 is used as a base system and thereto a relay station is added.
2. Description of the Related Art
A radio communication system carrying out communication with the use of a radio channel, i.e., WCDMA, CDMA2000 or such, as a typical example, currently spreads worldwide. In such a radio communication system, a plurality of radio base stations are set for a service area, and each radio terminal carries out communication with another communication apparatus (i.e., another communication terminal) with the use of any of the radio base stations. At this time, an overlapping area is provided with an adjacent service area in which an adjacent base station can carry out radio communication, and, handover to the adjacent radio base station is available when a radio environment degrades.
Further, as a radio communication method, for example, code division multiplexing, time division multiplexing, frequency division multiplexing, OFDMA or such, is applicable. In these methods, generally speaking, a plurality of radio terminals can connect to a single radio base station simultaneously.
However, even within the service area in which the radio base station can carry out radio communication, high speed communication may not be available in a place close to the area boundary, since the radio environment may not be satisfactory. Further, even within the area, radio signal propagation may be obstructed by a cause such as a building shade. Thus, an area (so-called dead zone) in which satisfactory radio connection with the radio base station is difficult may occur.
In order to solve the problem, a plan in which a relay station is disposed in the service area of the radio bases station, and radio communication is available between the radio terminal and the radio base station with the use of the relay station, has been proposed.
Especially, in a task group of 802.16j, introduction of such a relay station (RS) is currently studied.
As to the above-mentioned IEEE802.16, details are disclosed in, for example, IEEE Std 802.16TM-2004 and IEEE Std 802.16eTM-2005.